I'd be somewhat worried. CBU isn't structural. You might want to ask this question over at www.johnbridge.com. As a general rule, on a floor tile, you want a minimum of 80% thinset coverage (ideally, 100% support) which assumes a 100% subfloor. The biggest worry would be if someone in say high heels stood just in the right position...but see what the tile experts have to say on the referenced site.